The present invention generally relates to a double balanced mixer, and, more particularly, to a double balanced mixer used in mobile communication device such as portable telephones.
FIG. 1 is a schematic block diagram of a conventional mobile communication device 10, such as a portable telephone, that can perform analog and digital communications.
The communication device 10 is equipped with a mixer circuit 11 for digital communications and an amp circuit 12 for analog communications. The communication device 10 is also equipped with switches 14 and 15 for switching between the mixer circuit 11 and the amp circuit 12 in order to allow sharing of elements, such as antennas, between the analog and digital communications.
In digital communications, a control circuit 13 operates the switches 14 and 15 to active the mixer circuit 11. In this case, an oscillation signal output from an oscillation circuit 16 is input to the mixer circuit 11 as a carrier signal LO via the switch 14. The mixer circuit 11 generates an output signal RFout by mixing the carrier signal LO and a base band signal IF having an intermediate frequency. The output signal RFout is supplied to a subsequent circuit (not shown) via the switch 15.
In analog communications, the control circuit 13 operates the switches 14 and 15 to active the amp circuit 12. In this case, the oscillation signal output from the oscillation circuit 16 is input to the amp circuit 12 via the switch 14 as an input signal RFin. The amp circuit 12 generates the output signal RFout by amplifying the input signal RFin. The output signal RFout is supplied to the subsequent circuit via the switch 15.
The circuit area of the communication device 10 is increased by including both of the amp circuit 12 and the mixer circuit 11. Further, because the communication device 10 also requires the switches 14 and 15, its circuit area is further increased.
To make the communication device 10 compact, it would be advantageous to also use the mixer circuit 11 as an amp circuit in analog communications.
FIG. 2 is a schematic circuit diagram of the mixer circuit 11. The mixer circuit 11 is a double balanced mixer (DBM). The mixer circuit 11 includes transistors Tr1 to Tr6, resistors R1 to R3, and constant-current sources 21 and 22. The first and second transistors Tr1 and Tr2 form a first differential amplifier 23 in which both emitters are connected to each other. The collectors of the first and second transistors Tr1 and Tr2 are connected to a high potential power supply Vcc via the resistors R1 and R2.
The third and fourth transistors Tr3 and Tr4 form a second differential amplifier 24 in which both emitters are connected to each other. The collectors of the third and fourth transistors Tr3 and Tr4 are connected to the high potential power supply Vcc via the resistors R1 and R2.
The fifth and sixth transistors Tr5 and Tr6 form a third differential amplifier 25 in which both emitters are connected to each other via the resistor R3. The emitters of the fifth and sixth transistors Tr5 and Tr6 are connected to low potential power supplies (grounds GND) via the constant-current sources 21 and 22.
The collector of the fifth transistor Tr5 is connected to the emitters of the first and second transistors Tr1 and Tr2. The collector of the sixth transistor Tr6 is connected to the emitters of the third and fourth transistors Tr3 and Tr4.
As the mixer circuit, the base band signal IF is applied to the bases of the first to fourth transistors Tr1 to Tr4, and the carrier signal LO is applied to the bases of the fifth and sixth transistors Tr5 and Tr6. Then, the output signal RFout is output from the collectors of the first and third transistors Tr1 and Tr3 and the collectors of the second and fourth transistors Tr2 and Tr4.
As the amp circuit, a first control signal S1 is applied to the bases of the first and fourth transistors Tr1 and Tr4 and a second control signal S2 is applied to the bases of the second and third transistors Tr2 and Tr3. In this case, the first and second control signals S1 and S2 are set so that the first and second differential amplifiers 23 and 24 will be unbalanced.
For example, the first control signal S1 having an H level is applied to the bases of the first and fourth transistors Tr1 and Tr4 and the second control signal S2 having an L level is applied to the bases of the second and third transistors Tr2 and Tr3. Hence, the first and fourth transistors Tr1 and Tr4 turn on and the second and third transistors Tr2 and Tr3 turn off. Thus, the mixer circuit 11 operates as a differential amplifier by using the third differential amplifier 25. In other words, the fifth and sixth transistors Tr5 and Tr6 amplify the input signal RFin applied to the bases, and the output signal RFout is output from the collectors of the first and fourth transistors Tr1 and Tr4.
When the mixer circuit 11 operates as an amp, the two constant-current sources 21 and 22 are operating. However, the third differential amplifier 25 can be operated only by either the constant-current source 21 or 22. Accordingly, during amp operation, unnecessary current flows in the mixer circuit 11. Consequently, the power consumption of the circuit is greater than necessary.
It is an object of the present invention to provide a double balanced mixer having a small size and low power consumption.